move to toolchain3 for conda build toolchain

The old 'toolchain' package has been removed from the conda-forge
channel, and the 'gcc' package has no builds for Linux (several
non-authoratative sources say that it is "deprecated"), and it
seems that the toolchain3 package provides the necessary compilers

docker/kwant-latest.yml

@@ -13,9 +13,8 @@ dependencies:
     - blas #=1.1 openblas
     - openblas #=0.2.20
     # Build toolchain
-    - toolchain
+    - toolchain3
     - cython
-    - gcc
     - libgfortran
     ## Dev Dependencies
     # Testing

docker/kwant-stable-no-extras.yml

@@ -10,9 +10,8 @@ dependencies:
     - blas #=1.1 openblas
     - openblas #=0.2.20
     # Build toolchain
-    - toolchain
+    - toolchain3
     - cython
-    - gcc
     - libgfortran
     ## Dev Dependencies
     # Testing

docker/kwant-stable.yml

@@ -13,9 +13,8 @@ dependencies:
     - blas #=1.1 openblas
     - openblas #=0.2.20
     # Build toolchain
-    - toolchain
+    - toolchain3
     - cython
-    - gcc
     - libgfortran
     ## Dev Dependencies
     # Testing